Advances in electronics manufacturing technology have enabled the manufacture of inexpensive wireless devices in large quantities. Presently there is an interest in greatly expanding the use of wireless devices and networks of wireless devices beyond traditional communication uses.
A variety of new applications for networks including many low cost devices are contemplated. Such networks are generally expected to be what is termed Ad Hoc. In the context of wireless networks, Ad Hoc implies that when a number of devices are powered up, and at least some of the devices are in range of each other, the devices will interoperate to form a network. Ad Hoc networks can be formed without detailed planing by a network engineer. The placement of devices in an Ad Hoc network can be relatively random from the point of view of radio link considerations. The devices are placed as needed according to the use of the network. It may so happen that many devices are placed close together creating a potential for interference.
One category of application for networks that are to include many low cost devices is “Sensor Networks”. Sensor networks are typically envisioned to include many wireless devices coupled to sensors, and one or a few wireless devices coupled to data sinks. The data traffic in sensor networks is primarily from the sensors to the data sinks. There may be some small amount of control signal traffic from the data sink to the sensors. Thus, in sensor networks the foremost issues relate to sending messages from the sensors to the data sink(s).
The bandwidth available for use by any wireless network is limited by regulatory constraints. As the number of wireless devices in a given locale increases, the limits of bandwidth become problematic insofar as a finite bandwidth must be shared by all the devices in the network. In traditional cellular networks, bandwidth reuse is achieved by dividing the area in which the cellular network is deployed into a number of cells, the size of which is on the order of the range of mobile wireless devices, and assigning specific sub-bands to specific cells in such a manner that adjacent cells use different sub bands. However, such a system requires strategic placement of a number of cell sites in order to achieve frequency reuse, and is therefore not ideal for use in Ad Hoc networks. A number of methods have been used to share available bandwidth in wireless communication systems. These include Frequency Division Multi-Access (FDMA), Code Division Multi-Access (CDMA), and Time Division Multi-Access (TDMA). One protocol for sharing bandwidth that is based on TDMA is known as polling Media Access Control (MAC). In polling MAC protocols, under the direction of a network controller wireless device, each other wireless device in the network is allocated a particular time slot in which to transmit messages. In such systems, the network controller sends a poll signal to each wireless device at a prescribed time to trigger the particular device to transmit any messages that need to be transmitted. Unfortunately, in networks using a traditional polling MAC, the bandwidth that is effectively available to each device is inversely related to the number of devices in the network.
It is generally desirable to increase the effective bandwidth available to devices in wireless networks. In particular, it is desirable to increase the bandwidth available in Ad Hoc sensor networks.